The present invention relates to a rotor shaft and, more particularly, to a rotor shaft made of a steel containing 8 to 13% of Cr suitable for use as a rotor shaft of steam turbine.
In recent years, 12% Cr alloy steel is finding a spreading use as the material of rotor shafts of steam turbines, because of its high mechanical strength and toughness at high temperature range. The 12% Cr alloy steel, however, has inferior bearing properties, particularly wear resistance property. Consequently, various accidents are often experienced such as seizure of the bearing metal in support of the rotor journal, damaging of the rotor journals and bearings and so forth, due to jamming of foreign matter such as dust in the bearing.
To obviate this problem, Japanese Patent Laid-open Publication No. 16744/80 discloses a rotor shaft made of a 12% Cr alloy steel, with journal portions covered by a layer of a metal having good bearing properties formed by a metallizing, as well as a rotor shaft made of a 12% Cr alloy steel, with built up or raised journal portions formed by welding with a welding rod of a metal having good bearing properties.
The rotor shaft having the journal portions covered with the metal layer formed by metallizing, however, exhibits only a small affinity or bonding force between the metal layer and the base metal of the rotor. In addition, the metal layer formed by the metallizing tends to involve oxides and voids which seriously lower the tensile strength and toughness of the metal layer. Consequently, the metal layer formed by the metallizing is liable to be separated from the journal portion when a bending force is applied to the rotor shaft.
The rotor shaft having raised welded journal portions involves the problem that, in order to prevent the cracking in the built-up welded layer, it is necessary to strictly select the kind of welding metal used with respect to the welding condition relative to the base metal. This sometimes imposes a difficulty in the selection of the welding metal having good bearing properties. In other words, in some cases, the padding or build up metal is selected to meet the welding condition, at the cost of a poor bearing characteristics.
In general, in the rotor shaft assembly in which a coupling is fitted by shrink fit to a steam turbine rotor shaft for connection to the rotor shaft of a generator, there is a fear of cracking in the rotor at the portion thereof where the coupling is attached by a shrink fit.
Under these circumstances, there has been an increasing demand for a turbine rotor shaft having a coupling integral therewith and exhibiting a high reliability and good bearing properties.